Plasma deposition of thin layers on substrates

ABSTRACT

A method for producing thin layers of coating substances on substrates by plasma evaporation, comprising shaping a coating substance as a hollow body, passing a non-reactive gas through the body and subjecting the gas while in the body to an electromagnetic field to form a plasma. Coating material is vaporized from the interior of the body by the plasma, entrained in the gas and carried by the gas to a substrate on which it is deposited in a thin layer.

United States [Patent [191 I Cakenberghe PLASMA DEPOSITION OF THINLAYERS ON SUBSTRATES [75] Inventor: Jean Lear Van Cakenberghe, Mons,

Belgium [73] Assignee: Compagnie Industrielle Des TelecommunicationCit-Alcatel, Paris, France [22] Filed: Apr. 20, 1972 [21] Appl. N0.:246,019

[30] Foreign Application Priority Data Apr. 27, I971 Belgium 7 66 3 45[52] US. Cl.. 117/93.l GD, 117/106 R, 117/106 A, 204/164 [51] Int. Cl.C23c 11/08 [58] Field of Search 117/93.1 GD, 93.1 CD, 106 R, 117/106 A,106 C, 107, 107.1; 204/164 [56] References Cited UNITED STATES PATENTS3,437,511 4/1969 Hough 1l7/93.1 GD

[ 1 Apr. 2, 1974 10/1969 lng et al. ll7/93.1 GD

FOREIGN PATENTS OR APPLICATIONS 1,142,262 1/1963 Germany 117/93.1 GD

Primary Examiner-William D. Martin Assistant Examiner-John H. NewsomeAttorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak [5 7ABSTRACT A method for producing thin layers of coating substances onsubstrates by plasma evaporation, comprising shaping a coating substanceas a hollow body, passing a non-reactive gas through the body andsubjecting the gas while in the body to an electromagnetic field to forma plasma. Coating material is vaporized from the interior of the body bythe plasma, entrained in the gas and carried by the gas to a substrateon which it is deposited in a thin layer.

2 Claims, 3 Drawing Figures PAIENTEIJAPR 2 mm 3801; 355

sum 1 or 2 PATENTEIJAPR 2 I974 3.801; 355

v sum 2 or 2 PLASMA DEPOSITION OF THIN LAYERS ON SUBSTRATES BACKGROUNDOF THE INVENTION 1. Field of the Invention The present inventionconcerns a method enabling thin layers of mineral substances to bedeposited, as well as the device for implementing the method.

2. Description of the Prior Art Thin layers are usually produced byevaporation in a vacuum or by a method called reactive projection. Thefirst method can be used only in cases where the substance to bedeposited decomposes when it is brought to a high temperature in avacuum, into elements having very different vapor pressures and the mostvolatile of which can have a vapor pressure which can be measured at thedepositing temperature. Such is the case, more particularly, with themajority of oxides, certain sulphides as well as of gallum arsenide andgallium phosphide.

The second method mentioned above consists in causing the evaporation ofthe material to be deposited in an electrical discharge at low pressure,between two electrodes one of which consists of the material to bedeposited or the metallic component of that material, the othercomponent then being contained in gaseous phase. The material to bedeposited is deposited in the form of a thin layer on a substrate,arranged at a few centimeters from that electrode, which can be incontact or otherwise with the second electrode. In the case where a thinlayer of zinc oxide, for example, is to be deposited, the firstelectrode can consist either of zinc oxide or of metallic zinc with apure gaseous oxygen atmosphere or an atmosphere consisting of oxygenmixed with a neutral gas such as argon.

This second method can certainly be used for the above-mentionedsubstances, but it is unsuitable for semi-conductor materials, for thethin layers thus obtained consist of very small micro-crystals so thatcertain electrical properties such as the mobility and service life ofthe charge carriers are subjected to detrimental influence. Moreover,this second method is characterized by a relatively considerabledissipation of energy and a relatively low depositing speed which can,moreover, vary within wide limits.

The object of the invention is therefore a method for depositing thinlayers which does not have the abovemetnioned disadvantages.

It also provides a device for producing thin layers, either oninsulating supports or on electrically conductive or semi-conductivesupports.

Lastly, it provides a device enabling thin layers of material havingelectrical, semi-conductive, piezoelectrical, magnetic and/or opticalproperties, as well as thin layers of material having a high meltingpoint such as refractory materials to be produced.

SUMMARY OF-THE INVENTION The method enabling thin layers to be depositedin a vacuum on the surface of a substrate arranged facing the opening ofa cavity in which a gas is injected at a pre-determined pressure ischaracterized in that a plasma is formed inside the cavity previouslylined on the inside with the substance to be deposited.

The method also enables layers to be deposited ont the surface of asubstrate when the cavity consists directly of the substance to bedeposited.

The device implementing the method according to the invention ischaracterized in that it comprises, on the one hand, a high-frequencyexcitation means generating an electromagnetic field, and, on the otherhand, inside a vacuum container, at least a substrate support, asubstrate, a cavity lined on the inside with the substance to bedeposited and having an opening facing the substrate and a means forinjecting a gas at a predetermined pressure into said cavity, promotingthe forming of a plasma within said cavity where there is theelectromagnetic field.

The device implementing the method according to the invention is alsocharacterized in that the support for the substrate comprises anelectrical heating means enabling the substrate to be brought to apredetermined temperature.

In aparticular embodiment, the device according to the invention ischaracterized in that it comprises, moreover, an electrode in thecavity, this electrode being connected to an appropriate electricalpotential so as to produce a spark suitable for causing the starting upof the plasma.

The cavity has, to great advantage, a cylindrical shape, the cylindricalwall being provided, on its inside, with longitudinal ribs. Moreover,the insulating container may, to great advantage, be cooled.

The invention will be described herebelow with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectionalview of an embodiment of the device according to the invention.

FIG. 2 is a transversal sectional view of an embodiment of the cavityhaving a great advantage, used in the device according to the invention.

FIG. 3 is a longitudinal sectional view of another embodiment of thedevice according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to FIG. 1, acylindrical chamber 2 forming a cavity, whose wall consists of, or islined on the inside with the substance to be deposited, is arrangedinside the tube 1, made of quartz or ceramic material, for example. Oneof the transversal faces of the cavity is provided with an opening 3.The high-frequency excitation device consists, here, of an inductionwinding 4 surrounding the tube 1 at the level of the cavity. Thiswinding is connected to a high frequency voltage supply 5. A substratesupport 6 is placed so as to have a substrate 7 facing the opening 3 inthe chamber 2. The electrical heating device 8 enables the substrate 7to be brought to a required temperature. In the embodiment shown by wayof an example, the substrate support 6 is arranged so as to be able topivot about an axis 9 in order to bring several substrates successivelybefore the opening 3.

As shown in FIG. 2, the cylindrical wall of the cavity comprises, on theinside, longitudinal ribs 10 so as to reduce the transmission of heatthrough the wall.

The operation of this device is as follows:

A gas is injected into the cavity through the duct 11 so as to producean atmosphere at a pre-determined pressure therein. When a highfrequency current through the induction winding 4, the electromagneticfield it induces inside the cavity forms a plasma thereon. The dischargewhich takes place in the plasma causes a great increase in thetemperature of the inside wall of the cavity, this producing adistilling of the inner wall and the establishing of a vapor pressure ofthe substance to be deposited. This distilled substance escapes throughthe opening 3 and is deposited on the substrate 7. In the arrangementaccording to the invention, the plasma is confined inside the cavity.

It has been noted that the thin layers thus obtained consist of crystalswhich are appreciably larger and better formed than those obtained byreactive projection. It has also been noted that the crystallinedirection of the thin layers is perfect.

In this device according to the invention, the walls of the cavityconstitute a thermal screen. In certain embodiments, the latter havebeen reinforced by arranging a second cavity round the first. Thisscreen effect enables the energy dissipated in the plasma to beincreased so as to bring the inside surface of the cavity to a very hightemperature in the order of several thousands of degrees without dangerfor the insulating tube 1.

In the particular embodiment shown in FIG. 1, the device comprises,moreover, an electrode 12 in the opening 3 formed in the chamber 2. Thiselectrode 12 is connected to an appropriate electrical potential supplyV so as to produce a spark suitable for promoting the starting up of theplasma.

In a varied version of an embodiment, the tube 1 is surrounded by acooling funnel. It is thus possible to obtain high evaporating speedsand relatively high vapor pressures inside the cavity, this promotingmolecular combination.

In a particular example of an embodiment, a cylindrical cavityconsisting of zinc oxide, 50 mm in diameter and 60 mm in height, hasbeen placed in a quartz tube. An induction winding consisting of threeturns made of copper tubing 6 mm in diameter, connected to ahighfrequency power generator, has been arranged about the tube, on thelevel of the cavity.

After having produced a vacuum in the order of 10 mm Hg in the tube 1,and after having heated the substrate to a temperature of 200 C, oxygenhas been injected in the cavity in order to produce a pressure in theorder of 5.10 mm Hg therein. The pressure in the container in which thesubstrate is placed is appreciably lower subsequent to the loss of headat the outlet of the cavity.

After having started up the high-frequency generator so that it suppliesa power of 4 kw at 3 mc/s, the rated power is reached after barely a fewminutes, and the zinc oxide is then deposited on the substrate in theform of a thin layer which has reached a thickness of 0.5 micron in 1minute.

According to another form of the invention, the induction meansimplemented to generate the plasma inside the cavity is placed in thevacuum about the cavity. Various precautions are taken in that case toavoid the pollution of the substrate. This embodiment, shown in FIG. 3,comprises a cylindrical chamber forming a cavity placed in a vacuumcontainer shown in the figure only by its base 30. This chamber 20 islined inside with 6 the material 21 to be sprayed, it comprises, at itsupper part, a central opening 22, and at its lower part, a gas inlet 23.The lateral face 24 of that cylindrical chamber 20 is surrounded by theturns 25 of an induction circuit 26, fed by a HF supply, not shown,arranged outside the container. This induction circuit 26 consists of ahollow conductor internally cooled by a water circuit 27, 27. Theinduction circuit is held in position by an insulating base 28 fixed tothe base 30 of the container. The conductor forming the inductioncircuit is itself lined with a layer of protective insulating material29, made of teflon, in a series of experiments, and of glass in anotherseries of measurements. A protective screen 31 made of insulatingmaterial completes the protection of the substrate with respect to anypollution caused by the metal forming the induction circuit. A seal ring32 made of refractory material which is a bad heat conductor arrangedround the opening 22 of the cavity 20 provides a poor heat contactbetween the cavity 20 and the insulating screen 30 while providingsatisfactory sealing.

The embodiments described obviously have no limiting character, and,needless to say, varied versions may easily be conceived by the man inthe art. The excitation of the plasma in the cavity has, for example,also been obtained by means of a wave guide device. The

cavity has also been divided into fragments in certain cases, so as toenable a penetration of the electromagnetic field in the case of veryconductive or refractory substances.

The applicant has also produced a device in which the cavity is drilledwith several openings so that several substrates are coveredsimultaneously.

Moreover, the cavity has been divided into several compartments withoutan appreciable reduction in the depositing speed having been noticed.

It must be understood that the devices according to the invention may beused to great advantage, for producing thin layers of varioussubstances: piezo-electric, semi-conductive, optical, magnetic,insulating substances, materials having great dielectric constancy,refractory materials or compounds thereof having a high melting point.

What we claim is:

l. A method of vapor deposition of a substance onto a substrate as acoating under vacuum, said substance or a component thereof being asolid under the applied vacuum and at ambient temperature butvaporizable under said vacuum and temperature when subjected to particlebombardment by a plasma, said method comprising a. shaping saidsubstance or said component thereof to form a hollow body having aninlet and an outlet opening,

b. placing said hollow body. in a chamber under vacuum,

c. passing a gas through said hollow body,

d. subjecting said gas while within said hollow body to a high frequencyelectromagnetic field thereby creating a plasma of said gas within saidbody whereby the body is bombarded by ionic particles from said plasmaand evaporates to form a stream of vapor entrained in said gas, and

p as l sjhe su strate 9,..ll9 fi l ii l o utlet opening of said bodywhereby the vaporized substance of said body is deposited as a coatingon said substrate.

2. The method of claim 1 wherein the substance of the coating consistsessentially of the material of said hollow body and the gas used to formthe plasma for vaporizing the material is non-reactive with saidsubstance.

2. The method of claim 1 wherein the substance of the coating consistsessentially of the material of said hollow body and the gas used to formthe plasma for vaporizing the material is non-reactive with saidsubstance.